The present invention relates to solid state image sensors, and more particularly to solid state image sensors for reproducing high definition images.
Charge transfer devices (CTD) such as charge coupled devices have been known as solid state image sensors adaptable for standard television systems, for example, the National Television Systems Committee (NTSC) system, and have found a variety of applications. In the NTSC system, the number of horizontal scanning lines is 512, the scanning system is of an interlacing scanning type with two fields for one frame, and the aspect ratio is 3:4. The number of picture elements of a CCD adaptable for the standard TV system, for example, in an interline transfer type CCD (IT-CCD) is approximately 500 (vertical).times.400 (horizontal). It is also reported that the number of horizontal scanning lines is planned to increase to a level of 1,000 or more, for example, 1,125.
The solid state image sensor such as the IT-CCD is superior to an image pick-up tube in many respects. For example, the image sensor is small in size, light in weight, and high in reliability. Further, it is essentially free from pattern distortion, sticking, and so forth. Having such excellent features provides for an increasing number of applications in many fields, for example, ITV, small commercial video cameras, etc.
In the IT-CCD, signal charges stored in a photodiode array are simultaneously transferred to the adjacent vertical CCDs (V CCDs) disposed in the photodiode array or photosensing region. Then, the signal charges are read out from a horizontal CCD (H CCD), through a respective readout cycle. During the read out from the chip, the next integration of signal charges is also carried out. Conventionally, signal charges from every other photodiode along the V CCD or signal charges from photodiode pairs are read out in the first field. And in the second field, the signal charges from the remaining photodiodes or photodiode pairs are read out to complete one frame image. The readout of signal charges of the second field is performed after the signal charges of the first field are completely read out from the chip. The readout is synchronized with the interlacing scanning on the TV. Therefore, each field has 256 lateral scanning lines and the fields are interposed between each other, based on different sampling points, i.e., different image sampling modes of the incident light image. The operation of reading out from every other photodiode is called "frame integration." On the other hand, reading out from pairs of photodiodes is called "field integration."
As for the IT-CCD, various studies have been reported. One of the studies relates to incorporation of a storage region into the IT-CCD chip. ("A New Configuration of CCD Imager with a Very Low Smear Level -FIT-CCD Imager," K. Horii et al. IEEE Transactions on Electron Devices. Vol. ED-31, No. 7, July, 1984).
By introducing a CCD storage region between the photosensing region and the H CCD region for temporary storage of signal charges, the mixing of smear charges diffused from the depletion layer beneath the photodiode region with signal charges transferring in the V CCDs is prevented. This is because the data output speed from the V CCDs is not restricted by the scanning speed of the lateral scanning lines of the TV. In other words, the sweep out of signal charges from the V CCDs can be accomplished at high frequency.
In the aforementioned report, the storage region contains two parallel one-half length vertical CCDs for each V CCd in the photosensing region. The parallel arranged two V CCDs in the storage region accept the signal charges from the corresponding V CCD, by switching a selective gate at every other signal charge transferred from the V CCD. However, because only the signal charges corresponding to one sampling point group are read out from the CCD chip in one field, as mentioned, each vertical CCD in the storage region only has a memory capacity sufficient to accommodate one-half of the signal charges in the corresponding V CCD of the photosensing region. Further, in this instance, all the vertical CCDs in the storage region are driven with the same timing. Thus, the image sampling mode does not change during the readout cycle from the chip.
However, there are still further problems remaining in the image sensors. One important problem is the resolution in the present device. FIG. 1 shows a color filter arrangement formed on a conventional frame integration type IT-CCD. In FIG. 1, each filter is colored green (G), blue (B) or red (R), and is respectively formed on one element including one photodiode. In an odd numbered field, the signal charges from every other row (horizontal) of the photodiode array are read out from the chip. Successively, in the even number field, the signal charges from the rest of the rows of the photodiode array are read out. ("Interline Imaging Type Single Plate Color Camera," K. Ooi et al, Journal of Television Institute, P. 111-112, 1981 (Japan)).
As will be seen in FIG. 1, with attention to the readout rows of odd or even numbered fields, the signal charges corresponding to green are obtained, jumping one cell in the horizontal direction in each field. However, signal charges of blue and red are only obtained in each field at every fourth photodiode along the horizontal direction. For the vertical direction each color arises every fourth photodiode in one field. Besides, this structure is so designed as to form a luminance signal based upon the relation Ye=2G+1/2(R+B) based on the color signals. After performing the scanning of the odd number field, the even number field is scanned according to the signals from the photodiodes.
In FIG. 1, the minimum unit necessary to complete one frame is composed of 4.times.4 elements. The rough arrangement for color filters leads to a moire effect and the appearance of dummy signals. Also, it is readily understood that the low resolution power which depends on the area of the minimum unit of the color filters is considerable.
These problems occur for the reason that conventional CCD chips are unable to read out the signal charges with different image sampling points, in one field.